In the preparation of polymeric materials, particularly mass processable copolymers, considerable amounts of unreacted monomers and low molecular weight polymers remain entrained in the copolymer product after the polymerization process. Additionally, it is common practice to employ various inert solvents and other volatile agents in the polymerization reaction mixture in order to achieve desirable operating conditions and characteristics of the polymer product. However, the presence of these low molecular weight contaminants in the final polymer product may have adverse effects on the polymer properties. Additionally, these volatile components are subject to leaching, thereby deterring use of the polymer product in food packaging applications. Accordingly, removal of these undesirable volatile components is highly desirable.
In order to produce the highest quality product, it is desirable to reduce the residual concentration of volatiles in the finished polymer as much as possible. For many commercial applications, including food and beverage packaging, federal regulations impose exacting standards on acceptable levels of volatiles in polymers comprising the package material. Thus, much effort has been expended in designing and developing devolatilization apparatus and procedures for reducing volatiles in the products of polymerization processes. An overview of devolatilization technology, is provided by Ramon Albalak, Polymer Devolatilization, (Marcel Dekker, Inc. New York, 1996), incorporated herein by reference.
One method of reducing volatiles in polymers is extrusion. In an extrusion method, the polymer is heated and pushed through an extrusion die. The extrusion process increases the surface area of the polymer allowing the heated volatile components to escape. Extrusion devolatilization may be coupled with steam stripping to remove additional volatiles. Problems associated with extrusion devolatilization include introducing the polymer to additional thermal and shear stresses. Such stresses contribute to the undesirable formation of gels and colored bodies. In addition, extrusion methods require intensive capital investment in large-scale extruders. Steam stripping can also add undesirable heat history to the polymer product stream.
Another problem with extrusion is that this technique may not sufficiently reduce volatile content, especially when the process stream contains only 15–30% polymer and the remainder being monomer and solvent. In order to remove volatile components such that the percentage of polymer is 90%–100% without adding significant heat history, the throughput from extrusion devolatilization may be unacceptably low.
Thus, what is needed in the art is a method for devolatilizing a polymer that is less capital intensive yet suitable for removing of volatiles from a polymer stream and does not introduce undesirable thermal and shear stresses.